Position control and monitoring circuit and method for an electric motor

ABSTRACT

A position control and monitoring circuitry is provided for an electric motor having a plurality of coils and a movable element. A drive circuit generates a square wave drive signal of variable mark/space ratio to drive the coils to position the movable element. A monitoring circuit modulates square wave transitions at a sub-harmnonic of the drive signal frequency and generates a monitoring signal for injection into the coils. A sensor senses the monitoring signal from the coils to determine the position of the movable element. A difference circuit drives the drive circuit with a difference between an input signal and an output signal of the sensor.

The present invention relates to means for driving and controlling anelectric motor. Particularly, but not exclusively the invention relatesto means for driving and controlling a linear electromagnetic motor,which comprises an integral position measurement system providinginformation on the output movement produced and enabling servo positioncontrol. A motor of this type is described in a prior GB PatentApplication 9517603.8.

The control electronics required by such motors must provide power driveto the device, in order for it to produce mechanical output power, andmust simultaneously provide precision excitation signals for theintegral position measurement system. In addition a means of extractingand signal conditioning the resultant position signal and forimplementing the servo control loop is required. The performance.efficiency, and cost of manufacture of systems employing such actuatorsis critically dependent on the concept and design of the associateddrive and control electronics.

FIG. 1 of the accompanying drawing shows a linear motor and integralposition measurement system of the type to which this invention isparticularly relevant, and FIGS. 2a, 2b and 2c show the equivalentcircuit or electrical schematic representations of the motor. Thefigures show the motor to consist basically of two or four inductivecircuit elements which are connected in a half bridge according to FIGS.2a and 2b or full bridge configuration as in FIG. 2c. DC or quasi-staticDC drive voltage is applied across the bridge in combination with asuperimposed steady state AC excitation signal to provide energizationfor the position measurement function. The resultant position signalappears as a single ended output at the node of the half bridge circuitor as a differential output at the mid point of the full bridgeconfiguration. Imbalance of the bridge is bought about by movement ofthe magnet and pole piece assembly--which creates differentialincremental changes in the inductance of the windings. As described inGB Patent application 9517603.8, interconnection of the windings isarranged such that the forces produced on the magnet assembly by currentflow in the coils contribute in unison, whilst the incremental changesin inductance are lumped according to their sign in the relevant arms ofthe half bridge or full bridge circuits.

The present invention sets out to provide a combined power drive,transducer excitation and signal conditioning system and positioncontrol loop, which performs simultaneously to maxirnise the overallelectromechanical efficiency of the system and to realise thecapabilities of the integral position measurement of the actuator.

According to the invention there is provided an electronic system,comprising a power amplifier with antiphase outputs for driving theactuator/transducer assembly, a pre-amplifier for receiving outputsignals from the actuator/transducer assembly, an input command signal,and an excitation and signal conditioning circuit which functions tocombine the outputs of the pre-amplifier, the input command signal, anda periodic excitation signal to provide input signals to the poweramplifier for driving the actuator/transducer assembly.

The excitation and signal conditioning circuit may comprises an errorconditioning and converter stage, which provides input to the poweramplifier and a transducer excitation function generator, which providesperiodic signals for combination with the input signals to the poweramplifier.

Preferably the excitation and signal conditioning circuit includes astage which transforms the input command signal supplied to the systemto a suitable form for feeding a summation circuit, which derives aposition error signal; and that the pre-amplifier supplies theactuator/transducer output signal to a second input of the summationcircuit. The power amplifier may be a switching power amplifier.

The input command signal may be converted to a proportional AC signalbefore being compared with a position feedback signal, alternatively theactuator/transducer output signal may be converted to a proportional DCsignal and compared directly to the input command signal.

The input signals provided to the power amplifier are preferably in theform of a pulse-width modulated representation which may be modified byadding a digital advancing/retarding technique to achieve a synchronousperturbation component.

The error conditioning and conversion circuit may also comprise: asynchronous detector, for demodulating incoming position errorinformation using a synchronous reference signal provided by the saidtransducer excitation function generator. a servo loop compensationstage, for receiving a signal from the synchronous detector andresponding to the DC component thereof by providing the compensationnecessary for the desired position control loop characteristic; aconverter stage, for converting a compensated DC error signal receivedfrom the servo loop compensation stage to a PWM signal, having period T;and a digital modulator for applying advance and retard to the PWMsignal received from the converter stage, to produce a modulation havinga period 2T.

The power amplifier may also be a linear amplifier and substantiallysinusoidal perturbation signals may be employed to energise the actuatortransducer assembly.

Preferably the excitation and signal conditioning circuit includes astage which converts the input command signal to balanced complementaryDC reference signals which are proportional in amplitude to the inputcommand signal and a first synchronous demodulator which receivesrespective signals from the amplifier and also receives a synchronousreference potential and produces DC outputs in proportion to thesynchronous component of the AC energising signal in the respectiveamplifier outputs.

The excitation and signal conditioning circuit may also comprise firstsumming means for summing the respective outputs of the said firstsynchronous demodulators with the said balanced complementary DCreference signals to form error signals and compensators for performingcontrol loop compensation on the respective error signals associatedwith the synchronous perturbation component of the power amplifieroutput.

In a first and second preferred embodiment of the invention, thetransducer excitation function generator and power amplifier areimplemented using switching techniques, this offering high efficiencyand drive power, and the signal conditioning processes are hybrid--thatis they employ mixed analogue and switching technology.

In two further embodiments of the invention, a linear power amplifier isemployed and steady state sinusoidal or near sinusoidal transducerexcitation is used. In these embodiments much of the signal processingis linear although certain hybrid techniques are utilised. These furtherembodiments offer the potential for greater precision of performance ofthe overall system, but lack the benefits of efficiency, compactness andrelatively low cost of manufacture of the preferred embodiments.

The invention will be further described by way of these exemplaryembodiments and with reference to the accompanying drawings in which:

FIG. 1 is a cross sectional diagrammatic view of a combined linearelectric motor and position measurement system with which the presentinvention can be used;

FIGS. 2a, 2b and 2c show electrical schematic circuits of the system ofFIG. 1;

FIG. 3 is an overall block schematic diagram of the electronic system inaccordance with the present invention arranged for achieving powerdrive. position measurement and position control of the motor of FIG. 1;

FIG. 3a is an overall block schematic diagram of a first preferredembodiment of the electronic system in accordance with the presentinvention employing a switching power amplifier and digitally derivedtransducer excitation signals.

FIG. 3b is an overall block schematic diagram of a second preferredembodiment of the electronic system in accordance with the presentinvention employing a switching power amplifier and digitally derivedtransducer excitation signals.

FIG. 4 is a schematic representation of a switching implementation ofthe power amplifier of FIGS. 3a and 3b, for providing power drive andtransducer excitation for the system of FIG. 1;

FIG. 5 is a timing diagram showing the means for achieving transducerexcitation in the electronic systems of FIGS. 3a and 3b by the use ofdigital modulation of the power amplifier input signal;

FIG. 6a is a block schematic diagram showing the principal sub-systemsof the error signal conditioning and converter stage of the embodimentof FIG. 3a;

FIG. 6b is a block schematic diagram showing the principal sub-systemsof the error signal conditioning and converter stage of the embodimentof FIG. 3b;

FIG. 7 is a block schematic diagram showing the principal sub-systems ofa motor energization and position control system defining a thirdembodiment of the invention

FIG. 8 is a block schematic diagram showing a motor energization andposition control system defining an alternative functional arrangementof the embodiment of FIG. 7.

In the general system block diagram of FIG. 3 an actuator/sensor unitsuch as that depicted in FIG. 1 is shown being driven by a poweramplifier 1. A single ended position signal e₃ and/or differentialposition signals e₄, e₅ derived from the actuator/sensor unit are fed toa pre amplifier 5 which includes a signal converter circuit. An inputsignal converter stage 4 produces a proportional version e₆ of the inputcommand signal e_(in), and the output of the pre-amplifier and signalconverter stage 5, e₈, is subtracted from this. The difference, theposition error signal e₇, is fed to a signal conditioning and conversionstage 2. In this stage the position error signal e₇ is demodulated ifnecessary, compensated. and converted to a signal suitable for feedingto the power amplifier 1. Compensated position error information andsynchronous transducer energization are combined in the output of stage2, the synchronous reference e_(ref) and digital modulation signals DMODrequired in the various embodiments of the invention for this purposebeing provided by an excitation function generator stage 3.

FIG. 3a is a block schematic of a first preferred embodiment of theinvention, in which an actuator/sensor unit such as that depicted inFIG. 1 is shown being driven by a switching power amplifier 1. Singleended position signals e₃ or differential position signals e₄, e₅derived from the actuator/sensor unit are fed to the pre amplifier andsignal converter stage 5_(a). In this first preferred embodiment of theinvention, the output of stage 5_(a) is an AC signal, proportional tothe position signal e₃ or to the differential position signals e₄, e₅.It may be a filtered version of these signals as shown by the path P1comprising a pre amplifier 5_(a1) and filter 5_(a2), or it may simply bean amplified version of the signals e₃, or e₄, e₅ as shown by path P2which features only the pre amplifier 5_(a1).

In order for the DC position command signal e_(in) of FIG. 3a to becompared with the position feedback signal e_(8a), it is converted to asynchronous AC signal e_(6a). This is the function of the input signalconverter stage 4_(a) which performs conversion of e_(in) to asynchronous AC signal e_(6a) using reference signal e_(ref) provided bythe excitation function generator 3_(a). In stage 4_(a), signal choppingor multiplying techniques are employed to produce an AC signalproportional in magnitude to the input e_(in) and proportional in phaseto the sign of e_(in). The frequency of the resultant AC positioncommand signal e_(6a) is equal to that of the position feedback signale_(8a), and the phase. as determined by the synchronous referencee_(ref) is arranged such that effective differencing of the AC positioncommand signal e_(6a) and the AC position feedback signal e_(8a) can beperformed by the summation stage 6_(a).

The resultant output e_(7a) of the summation stage 6_(a) of FIG. 3a isthus a synchronous AC signal expressing in magnitude the degree ofposition error yielded by the control loop, and in phase, relative tothe reference signal e_(ref), the sign of the position error. As shownin FIG. 3a, the position error signal is then fed to the signalconditioning and converter stage 2a, the function of which is todemodulate the AC position error signal, to provide a position errorsignal suitably compensated for achieving closed loop stability in theposition control system, to convert this signal to a pulse widthmodulated (PWM) representation suitable for driving the switchingamplifier 1, and finally to modify this PWM representation such as toadd a synchronous perturbation component which serves as energizationfor the position measurement function of the sensor/actuator unit ofFIG. 1.

The block diagram of FIG. 6a shows the principal sub-systems of thesignal conditioning and conversion stage 2_(a) and of the input signalconvener stage 4_(a) appropriate to the first preferred embodiment ofthe invention. Incoming position error information e_(7a) is demodulatedby the synchronous detector 2_(a1) using the synchronous referencesignal e_(ref) provided by the transducer excitation function generator3_(a) (see FIG. 3), and the resultant signal is fed to the servo loopcompensation stage 2_(a2). This stage responds to the DC component ofthe demodulated signal providing the compensation necessary for thedesired position control loop characteristic. Following this, thecompensated DC error signal is converted to a pulse width modulated(PWM) signal of period T by converter stage 2_(a3). This signal in turnfeeds digital modulator 2_(a4) which applies synchronous advance andretard to the T period PWM signal to produce a 2T period modulation. Thenecessary digital control signals for this digital modulation, DMOD, areprovided by the excitation generator 3_(a). In this first preferredembodiment of the invention, the input position command signal e_(in) isconverted to a synchronous AC signal in order for it to be summed withthe transducer position signal. This is shown in FIG. 6a where the inputsignal converter stage 4_(a) is seen to comprise an input bufferamplifier 4_(a1) which feeds a chopper or multiplier stage 4_(a2). Thechopper stage uses reference signal e_(ref) to multiply or to chop theinput command signal e_(in) to produce an AC signal e_(6a) which isproportional in magnitude to e_(in), but modulated by e_(ref) such as toallow effective summation of e_(6a) with the position feedback signale_(8a) at the synchronous frequency 1/2T.

FIG. 3b is a block schematic diagram of a second preferred embodiment ofthe invention, in which an actuator/sensor unit such as that depicted inFIG. 1 is shown being driven by a switching power amplifier 1. Singleended position signals e₃ or differential position signals e₄, e₅,derived from the actuator/sensor unit are fed to the pre amplifier andsignal converter stage 5_(b). In this second preferred embodiment of theinvention, the output e_(8b) of stage 5_(b) is a DC signal proportionalin magnitude to the position signal e₃ or to the differential positionsignals e₄, e₅, and in sign to the relative phase of the positionfeedback signals to the synchronous reference e_(ref). In thisconfiguration, the converted position feedback signal e_(8b), being a DCproportional signal, is compared directly to the DC input command signale_(in), avoiding the need to convert the command signal e_(in) to asynchronous AC signal. This further avoids the requirement to demodulatethe error signal e_(7b) produced by the summer stage 6_(b). Thus theinput signal converter stage 4_(b) is simply a DC buffer amplifier whichfeeds a scaled version e_(8b) of the DC input command signal e_(in) tothe summer stage 6_(b).

The resultant output e_(7b) of the summer stage 6_(b) of FIG. 3b is thusa DC signal expressing directly in sign and magnitude the degree ofposition error yielded by the control loop. As shown in FIG. 3b, theposition error signal is then fed to the signal conditioning andconverter stage 2_(b), the function of which is to provide a positionerror signal suitably compensated for achieving closed loop stability inthe position control system, to convert this signal to a pulse widthmodulated (PWM) representation suitable for driving the switchingamplifier 1 and finally to modify this PWM representation such as to adda synchronous perturbation component which serves as energization forthe position measurement function of the sensor/actuator unit of FIG. 1.

The block diagram of FIG. 6b shows the principal sub-systems of thesignal conditioning and conversion stage 2_(b) and of the input signalconverter stage 4_(b) appropriate to the second preferred embodiment ofthe invention. Incoming DC position error information e_(7b) is fed tothe servo loop compensation stage 2_(b1), this stage providing thecompensation necessary for the desired position control loopcharacteristic. Following this, the compensated DC error signal isconverted to a pulse width modulated (PWM) signal of period T byconverter stage 2_(b2). This signal in turn feeds digital modulator2_(b3), which applies synchronous advance and retard to the T period PWMsignal to produce a 2T period modulation. The necessary digital controlsignals for this digital modulation, DMOD, are provided by theexcitation generator 3_(b). In this second preferred embodiment of theinvention, the input position command signal e_(in) requires noconversion to a synchronous AC signal in order for it to be summed withthe transducer position signal. Thus in FIG. 6b input signal converterstage 4_(b) is seen to comprise only a DC input buffer amplifier 4_(b1)which feeds the summer 6_(b).

FIG. 5 is a timing diagram, which shows the way in which the abovepreferred embodiments of the invention employ switch-mode techniques toprovide simultaneous power drive and precision transducer excitation fora combined motor and position sensor device such as shown in FIG. 1.

Wave form F₁ in FIG. 5 shows an equal mark/space ratio square wave formsuch as might be applied to the input of the switching amplifier 1 ofFIG. 3. The amplifier shown in diagrammatic form in FIG. 4 acts usingpower semiconductor devices F1, F2, F3 and F4 to switch the ends of theapplied load, in a fashion closely following the switching input waveform e₉, between the applied power rails 0 V, +VE. This achieves minimaldissipation in the amplifier 1 and has the benefit that the amplitude ofthe amplifier output and the relative times of the switching transitionsof the output are well defined and repeatable. The inductance of theload acts to smooth or average the current flowing and the magnitude ofany resultant steady DC component is determined by the mean appliedvoltage divided by the net load circuit resistance. Thus for wave formF₁ of FIG. 5, having an equal mark/space ratio, the average DC loadcurrent is zero corresponding to a zero input to the amplifier.

Wave form F₂ of FIG. 5 shows pulse width modulation (PWM) of theamplifier input signal, this being achieved in the embodiment of FIG. 4by time shifting the rising edge of the wave form, keeping the overallperiod to T seconds. The average DC value of the voltage applied to theload is now non zero and DC current flows in the load. Wave form F₃shows PWM modulation, again using the rising edge of the wave form, butthis time retarded to reverse the sign of the average current flowing inthe load compared to that of wave form F₂. This follows alreadyestablished principles, and it can be appreciated that PWM modulationcan be achieved by modulating either transition of the wave form in adiscrete (digital) or continuous (analogue) fashion. Either method wouldserve to provide DC or quasi static drive current to the motor system ofFIG. 1 in proportion to the position error or command signal.

Wave form F₄ shows how digital techniques are employed to achievesynchronous PWM modulation of the amplifier drive wave form and hencewell-controlled and synchronised AC excitation for the positionmeasurement function. Here digital modulation techniques are usedalternately to advance and retard a given transition of the PWM waveform by a fixed fraction of the PWM period T. This has the effect ofalternately offsetting the mean drive voltage applied to the actuator bya constant fraction and at a constant frequency. The period of thisapplied perturbation is exactly twice that of the pulse width modulationassociated with the quasi-static signal and the phase relative to agiven rising or falling transition of the reference wave form F₁ isconstant. These conditions are ideal for recovery of the transducersignal since the PWM modulation associated with DC or quasi staticcurrent is of period T whilst the excitation source for the positionmeasurement, and hence the position information, is conveyed by themagnitude and relative phase of a signal of period 2T. Using synchronousdemodulation techniques, the component due to the DC modulation atperiod T is automatically suppressed, allowing precision recovery of theposition information despite the presence of the relatively large higherfrequency PWM components.

This is illustrated by further reference to wave form F₇, which is theresult of synchronously sampling wave form F₄ with the 2T periodfunction wave form ₆. Wave form F₄ carries no DC component, but isdigitally modulated with a 2T period perturbation component. Synchronoussampling acts to multiply the wave form by a synchronous reference, inthis case wave form F₆, which, given the correct relative phaserelationship, acts to demodulate the synchronous component and yield aDC-proportional component in the resultant wave form. In the example ofwave form F₇, the resultant has a net positive DC component of fraction1/8 of the peak amplitude.

Wave form F₈ further shows how the detection method rejects the higherfrequency PWM components carrying the DC or quasi-static signal to thesystem, Here wave form F₅, which carries both 2T period perturbation andT period PWM modulation, is synchronously sampled by wave form F₆, theresultant being wave form F₈. Although more complex, the average is thesame as that of wave form F₇, this being contributed solely by the 2Tperiod component of the wave form F₅.

Thus the perturbation or transducer excitation signals are applied usingdigital techniques to achieve precise, repeatable and highly synchronousposition signals, and the DC or power drive signals are applied asadditional pulse width modulation using either analogue or digitaltechniques.

It can be appreciated that suppression of the higher frequency T periodPWM components, achieved by the use of synchronous demodulation, iseffective for all perturbation functions with periods being a binarymultiple of T.

FIG. 7 shows a second embodiment of the invention in which acomplementary output linear power amplifier 7₁ is employed to drive theactuator/sensor unit. The amplifier provides quasi static DC outputvoltage or current to provide power drive to the actuator/sensor unit,and superimposed sinusoidal or near sinusoidal functions for the ACenergising signals.

In this embodiment the amplitudes of the AC energising voltages drivingthe actuator/sensor unit are independently controlled in local closedloop feedback systems. this operating the position sensing system of theactuator/sensor unit in a high precision null balance mode.

Referring to FIG. 7, the outputs e₇₁, e₇₂ of amplifier 7₁ drive theactuator sensor unit which is configured either as a half bridge or fullbridge circuit producing the corresponding single ended position outputsignal e₇₃ or the differential position output signals e₇₄, e₇₅. Theseposition output signals are fed to a pre-amplifier stage 7₁₀ whichproduces a single ended position output signal e₇₆.

The amplifier outputs e_(7l), e₇₂, in addition to driving the actuatorsensor unit, are also fed to stages 7₃ and 7₄, these being synchronousdemodulators which, in conjunction with the synchronous referencepotential e_(ref), produce DC output signals e₇₉, e₇₁₀ which are inexact proportion to the synchronous component of AC energising signal inthe amplifier outputs e₇₁, e₇₂. These potentials are summed withreference signals e₇₁₅, e₇₁₆ which are derived from the position commandsignal e_(in) by an input signal converter stage 7₉. The stage 7₉ actsto convert the command signal e_(in) to balanced complementary DCoutputs e₇₁₅, e₇₁₆ which are proportional in amplitude to e_(in) but ofopposite sign and superimposed on a constant component of voltage.Signals e₇₁₁, e_(7l2) are the error signals or the deviations ofpotentials e₇₉ e₇₁₀ from the commanded values derived from e_(in). Theseerror signals are fed to stages 7₅, 7₆ which perform control loopcompensation to achieve the desired response of amplitude control of theAC energising component of signals e₇₁, e₇₂ Following compensation, theoutputs of stages 7₅, 7₆ are fed to voltage controlled functiongenerators 7₇, 7₈ which produce sinusoidal or near sinusoidal outputvoltages e₇₁₃, e₇₁₄ which are synchronous with the switching referencee_(ref) and proportional in amplitude to the outputs of the compensationstages 7₅, 7₆. The outputs of function generators 7₇, 7₈ are summed intothe inputs of amplifier 7₁ to complete the closed loops in which the ACenergising components of the actuator sensor drive potentials e_(7l),e₇₂ are controlled.

The AC energization component of the actuator sensor potentials are thusanti phase sinusoidal or near sinusoidal functions which are controlledto vary in amplitude in a complementary fashion, in proportion to themagnitude and sign of the control system input command signal e_(in). Inorder to provide for position sensing and control in mid range whene_(in) is zero, an offset or constant magnitude component is added tothe energising potentials. As mentioned above this component isintroduced by an input converter stage 7₉ in FIG. 7.

Returning to FIG. 7. It will now be evident that the signal e₇₆appearing at the output of pre amplifier 7₁₀ is a position error signal.The controlled amplitude AC energising potentials produced by the localcontrol loops described above are summed by the inductive bridge circuitof the actuator unit and, given that the relative imbalance of theenergising potentials corresponds to the relative imbalance of theactuator bridge inductances (brought about by movement of the actuator),then the position output signal e₇₃ or e₇₄, e₇₅ from the sensor/actuatoris of null amplitude.

To implement closed loop position control of the actuator sensor unitthe pre amplified null error signal e₇₆ is fed to a synchronousdemodulator stage 7₁₁. Using the synchronous reference e_(ref) thisstage produces a DC output e₇₇ which is proportional in magnitude andsign to the AC error signal e₇₆. This in turn feeds control loopcompensation stage 7₁₂ which acts on the error signal e₇₇ to provide thedesired response characteristics for the overall servo position controlfunction. The position control loop is completed by splitting thecompensated position error signal e₇₈ into complementary outputs viaamplifiers 7₁₃, 7₁₄ and summing these signals with the AC energisinginputs e₇₁₃, e₇₁₄ into the power amplifier 7₁.

The benefit of the null balance control loop described above is that forall output positions of the actuator unit according with the commandedposition represented by signal e_(in), the position output signals e₇₃or e₇₄, e₇₅ produced by the actuator unit are null. This means thatprecision of signal processing in the position control loop is notrequired.

FIG. 8 shows a third embodiment of the invention which is a modificationof the configuration depicted in FIG. 7. In this embodiment the ACenergising voltages driving the actuator/sensor unit are maintained atconstant amplitude with the result that the feedback signals from thesensor portion of the unit correspond to absolute position and not toposition error as in the system of FIG. 7 described above. Thisarrangement has the benefit of reduced overall complexity and hencecost, but lacks the precision of operation attainable from the system ofFIG. 7.

Referring to FIG. 8, the outputs of amplifier 8₁ drive theactuator/sensor unit producing a single ended position output signal e₈₃or differential position output signals e₈₄, e₈₅. These position signalsare fed to a pre amplifier stage 8₁₀ which in turn feeds a demodulatorstage 8₁₁. The demodulator stage, using the synchronous reference signale_(ref) converts the AC position feedback signal e₈₆ to a DC signal e₈₇,the magnitude of which is proportional to e₈₆, and the sign beingdetermined by the phase of e₈₆ relative to that of the referencee_(ref). To achieve position control, the demodulated position feedbacksignal e₈₇ is compared with a buffered version e₈₈ of input commandsignal e_(in) by the summation stage 8₅. The resultant position errorsignal e₈₉ is fed to the stage 8₆ which provides the necessarycompensation of the error signal for the required overall positioncontrol loop response. This compensated error signal e₈₁₀ then feeds thepower amplifier via an additional summer and driver stages 8₇, 8₈ and8₉. The additional summer stage 8₇ finctions to allow injection of thetransducer excitation unction e₈₁₁ into the power amplifier 8₁. This isa periodic sinusoidal or near sinusoidal function produced by stage 8₄.This function emerges from stage 8₇ summed with, and hence superimposedupon the position error signal e₈₁₀. This composite signal e₈₁₅ isconverted to balanced antiphase signals by the stages 8₈, 8₉ where itdrives the inputs of the power amplifier 8₁. The outputs of thisamplifier e₈₁, e₈₂ thus contain balanced or differential drivecomponents of both the power actuation and transducer energizationsignals required by the actuator/sensor unit. The function of theenergization control loop described here is to stabilise the amplitudesof the transducer energization component of the actuator/sensor drivesignals to a degree adequate for the required position accuracy andrepeatability. To achieve this, the outputs e₈₁, e₈₂ of the amplifier 8₁are fed to stage 8₂ which performs demodulation of the perturbationcomponent of e₈₁, e₈₂ using the synchronous reference signal e_(ref).The stage 8₂ responds to the difference of signals e₈₁, e₈₂ thusensuring that the overall amplitude of the energization applied to theactuator/sensor unit is taken into account. The output e₈₁₂ of stage 8₂is a DC signal proportional in magnitude to the synchronous component ofthe difference between e₈₁ and e₈₂ which is compared with a DC referencepotential Vref in the summer stage 8₁₃. The resultant error signal e₈₁₃feeds stage 8₃ which provides the compensation required to achieve thedesired response characteristic of the excitation signal stabilisationloop. The compensated DC output, e₁₈₄ then feeds the function generatorstage 8₄. This stage, using the synchronous reference signal, e_(ref) ;produces a sinusoidal output wave-form approximately proportional inamplitude to the compensated DC input e₈₁₄ and controlled in phase, orsynchronous with the reference e_(ref).

I claim:
 1. A position control and monitoring circuitry for an electricmotor having a plurality of coils and a movable element,comprising:drive means generating a square wave drive signal of variablemark/space ratio to drive the coils to position the movable element, thesquare wave drive signal having a drive signal frequency; monitoringmeans for modulating square wave transitions at a sub-harmonic of thedrive signal frequency to generate a monitoring signal for supply to thecoils; sensing means for sensing the monitoring signal from the coils tothereby determine the position of the movable element; and differencemeans for driving the drive means with a difference between an inputsignal and an output signal of the sensing means.
 2. A circuitryaccording to claim 1, wherein the monitoring means modulates the squarewave transitions at half of the drive signal frequency.
 3. A circuitryaccording to claim 1, wherein the square wave transitions are, in afirst sense, modulated by the drive means, and the square wavetransitions are, in a second sense opposite to the first sense,modulated by the monitoring means.
 4. A circuitry according to claim 2,wherein the square wave transitions are, in a first sense, modulated bythe drive means, and the square wave transitions are, in a second senseopposite to the first sense, modulated by the monitoring means.
 5. Aposition control and monitoring method for control and monitoring anelectric motor having a plurality of coils and a movable element,comprising the steps of:with a drive circuit, generating a square wavedrive signal of variable mark/space ratio to drive the coils to positionthe movable element, the square wave drive signal having a drive signalfrequency; modulating square wave transitions at a sub-harmonic of thedrive signal frequency to generate a monitoring signal for supply to thecoils; sensing the monitoring signal from the coils to thereby determinethe position of the movable element; and driving the drive circuit witha difference between an input signal and an output signal of the sensingstep.
 6. A method according to claim 5, wherein the step of modulatingthe square wave transitions is performed with a sub-harmonic having halfof the drive signal frequency.
 7. A method according to claim 5, whereinthe step of modulating is performed by modulating the square wavetransitions in a first sense by the drive circuit, and by modulating thesquare wave transitions in a second sense opposite to the first sense bythe monitoring signal.
 8. A method according to claim 6, wherein thestep of modulating is performed by modulating the square wavetransitions in a first sense by the drive circuit and by modulating thesquare wave transitions in a second sense opposite to the first sense bythe monitoring signal.